Exact determination of the vibration of a composite structure becomes impossible beyond the low-frequency range due to uncertainties in the structure. Therefore, the prediction model in the mid- to high-frequency range must include the effects of the irregularities. I will show how the power spectral density of an irregularity in a component can be included in the model. A lightweight floor/ceiling system is used as an example. Effects of twisting joists or varying stiffness of the floor show up as a simple summation in the original forward operator. Understanding random irregularities in composite structures have become important, because any manufacturing process introduces randomness, and traditional modelling techniques such as the finite element method or statistical energy analysis have not been very successful.